Signal expansion or compression



Jan. 311, 1959 GRAHAM 2,495,809

SIGNAL EXPANSION 0R COMPRESSION Filed Sept. 6, 1945 5 Sheets-Sheet 1 UVIV 20 I INVENTOR fimfige [C Grafiam BY YI Z/WW ATTORNEY Jan. 31, 1950 vG. K. GRAHAM 2,495,809

SIGNAL EXPANSION 0R COMPRESSION Filed Sept. 6, 1945 3 Sheets-Sheet 2 s;I Y

1 i .1 5 w w AMP aarpur M :l j 7 MAGA/t-T/C 3 i MAGNET/C MAHR/A L M 144TEE/AL INVENTOR (Zearge K Grafiam ATTORNEY Jan. 31, 1950 G. KJGRAHAM2,495,899

SIGNAL EXPANSION OR COMPRESSION Filed Sept. 6, 1945 3 Sheets-Sheet 3Jay/v0 WAVE 7 AMPL/F/ER 007 07 JOUND WAVE 49 I'l'l' ,55 48 1: T

INVENTOR Geazg ff. Grafiam ATTORNEY Patented Jan. 31, 1 950 SIGNALEXPANSION OR COMPRESSION George K. Graham, Oceanside, N. Y., assignor toRadio Corporation of America, a corporation of Delaware ApplicationSeptember 6, 1945, Serial No. 614,736

8 Claims.

This application relates to an improved method of and means forregulating the sensitivity of a transducer such as, for instance, amicrophone or phonograph pick-up element. By means of my invention thetransducers sensitivity is regulated inversely or directly in accordancewith the amplitude of the stimulus to which the transducer is subjected,so that compression or expansion of the currents representing the soundwave is accomplished.

In many radio systems sound waves are converted into electrical currentsand transmitted from one point to another by way of conductors such aslines or by radiation. The transmitted material may represent voice ormusic or television or as a matter of fact intelligence of any type andmay be put to various uses. In one particular application of myinvention sound waves are converted to electrical currents, amplified,and supplied as output to be used as desired. This use may includemodulation of a radiant energy transmitter or distribution to listeningpoints, etc.

In the transmission of program material the volume level range which istransmitted must lie between two extremes, the upper limit establishedby the maximum handling capacity of the transmission medium and thelower limit by the level of noise present. In the present art it iscustomary to reduce the level of the higher level passages to a safevalue and to increase the level of the lower level passages to overcomethe effect of noise.

An object of my invention is to regulate the level of program materialwithin predetermined limits. This object is attained by providing a newmethod of and means for regulating the sensitivity of the microphone,phonograph pick-up or other transducer supplying the material fortransmission.

Some present day compression and expansion amplifiers depend for theiroperation on the change in grid bias of a variable mu type of tube whichis provided by a rectified portion of the output of the compression typeamplifier. Using such a method, wide variations in grid bias due tolarge output level changes tend to shift the operating point of the tubein such a way as to cause serious distortion.

A further object of my invention is a better method of and means forcompressing or expanding the range of program material.

This improved regulation or control by compression or expansion isaccomplished in accordance with my invention by regulating thesensitivity of the transducer per se rather than of a network and/oramplifier to which the current, resulting from the conversion from soundwaves to electrical current, is supplied.

A further object of my invention is improved sensitivity control of atransducer of the nature described above.

An additional object of my invention is improved automatic control ofthe sensitivity of a transducer of the nature described above.

The above objects and others which appear hereinafter are attained inaccordance with my invention by utilizing some of the converted energyafter, if desired, amplification to regulate the sensitivity of themicrophone, phonograph pick-up, or transducer or other means forconverting from sound waves to electrical currents. Where compression isdesired, if the program level is low the control is such as to increasethe sensitivity of the transducer, while if the program level is highthe sensitivity of the transducer is reduced. Where expansion isdesired, if the program level is low the control is such as to decreasethe sensitivity of the transducer, while if the program level is highthe sensitivity of the transducer is increased. In an embodiment thefield strength is made non-uniform, with an intensity pattern such thatcompression or expansion as desired may be obtained.

My invention is applicable to transducers of various types, includingmicrophones and phonograph pick-up devices. In the detailed descriptionwhich follows, I have disclosed application of my invention totransducers of the electromagnetic type and of the electrostatic type.

In disclosing my invention in detail reference will be made to theattached drawings, wherein- Fig. 1 illustrates schematically anembodiment of my invention wherein the sensitivity of a transducer ofthe electromagnetic type is controlled. The transducer in the embodimentis a ribbon microphone.

Fig. 2 illustrates schematically a modification of the arrangement ofFig. 1. In Fig. 1 the sensitivity control is carried out by control ofthe field strength. In Fig. 2 the position of the pickup element in thefield is controlled.

Figs. 2a and 2b are modifications of the arrangement of Fig. 1.

Fig. 3 illustrates schematically the application of my invention to adifferent type electromagnetic microphone. In this embodiment themicrophone is of the dynamic type, having a diaphragm operated coilmoving in a magnetic field.

Fig. 4 shows my improved compression and expansion means as applied toan electrostatic 3 transducer, while Fig. is a modification of thearrangement of Fig. 4.

In the embodiments of Figs. 1 and 2, I have applied my invention to anelectromagnetic microphone wherein sound pressure is converted toelectrical energy by means of a moving sound pressure operated conductorwhich is caused to cut the magneticfield. In such a microphone the E. M.F. induced in the conductor is roughly proportional to the displacementof the conductor and to the strength of the magnetic field. The inducedE. M. F. for a given displacement of the conductor is greatest when themoving conductor cuts the greatest number 'of lines of magnetic flux,and is diminished as the conductor assumes a position in which it cuts asmaller number of lines of magnetic flux.

In Fig. l, the conductor or microphone ribbon I2 is located in the fieldof a magnet 14. The magnet 64 may be of the field winding type or of thepermanentmagnettype, the latter type-being shown. Current-inducedin theribbon misused to excite winding A to control the effectiveness(strength) -of the magnetic field, thereby to control-thetransducer ssensitivity. A compensating winding B, the operation of which will bedescribed-later, is'also-mounted on themagnet. In

the embodiment-of Fig. '2 I control the position of the ribbon -l2in-the magnetic field to-control the sensitivity of thetransducer.

In Fig. 1, l0 designates a transducer or microphone of the ribbon type.In this microphone a conductor-in the 'form of ,a ribbon l2, acted on bysoundwaves, isllocatedbetween the poles of a magnet M, which maybepermanent ormay be of the type having afield producing winding. Theribbon 12, as stated above, is locatedbetween the pole piecesand-connected in series with theprimary wiring of a transformer 16whereby the F.-inducedin the ribbon l2 during itszmovement by the soundwaveszisfed by lines !3 to unit l8. The ribbon conductor l.2 isv also inseries with a winding B,-the purpose of which appears hereinafter. Theunit l8 .may include an amplifier if desired and .theamplifier outputmaybe put to any use, such as being :fed to a loudspeaker ortransmitter. The magnet .lll also has mounted thereon a second coil .Awhich when energized varies .the magnetic field,thereby controlling thesensitivity of thetransducer. .Coil A is connected by apolarityreversingswitch 26 to the load impedance '22ofa dioderectifier 24. This coupling-may, if desired, include a direct current amplifier 25. The diode24hasits anode tapped to a potentiometer 30 across theoutput'of theamplifier I8.

The cathode of the diode viscOnnected to the low potential end of thepotentiometer resistancei30, so that a portion of Lthepctentiometer isincluded inseries with the load resisto1x22 and the cathode to anodeimpedance of the diode 24.

The potential across the load impedance 22'is proportional to the:magnitude of theprogram at the output of the amplifier 18, so that thecurrent passed throughcoil .A is proportional to the output voltageofthe amplifier l8. ".Under these .con-

ditions, the sound energy .or waves impinging on the microphone'ribbon12 will cause variations in through the coil causes a. magnetic flux tobe set up which opposes the normal flux in the magnet 14. Then anyincrease in sound pressure will cause a decrease in magnetic flux inwhich the conductor moves. This decrease in magnetic flux will cause areduction in the E. M. F. impressed on the input of the amplifier I8.Conversely, a decrease in sound pressure will cause an increase in themagnetic flux. Under these circumstances, the electrical output of themicrophone will be proportionately greater for low level sounds than forhigh level sounds. This will cause the louder passages of sound to becompressed. Reversing the switch 20 will reverse the above and causeexpansion of the loud passages. The diode circuit may have suitableelectrical networks including, for example, a condenser 23 so that thevoltage impressed on the coil A will follow the average level'of energyor be responsive to the peaks of energy, so that by a suitablecombination of circuit elements, the required degree of expansion orcompression can be achieved. If it is desired that the sensitivity becontrolled in accordance with the average level, then the condenser 23is large so that the network has a time constant which isrelativelylarge with respect .to the. average variations in volume intensity. Ifthe sensitivity is to be controlledby peaks only, then the time constantis made relativelysmall and condenser 23 Wouldhaveiless capacity.

Coil B is in series with .the moving conductor and is so arranged inposition and number of turns that any .E. .M. F. .induced in the movingconductor, due to the variations in the magnetic field under theinfluenceof the current in coil A, will be opposedby an equal E. .M. F.induced in .coil .B. This will minimizethe effect of any volt- .agewhich .might be induced in the moving conductordue to a change inmagnetic flux. Complete ,cancellation'ofzthe effect of the potential set11p in.A appearspossible.

.In practice in many .cases, my invention will be appliedwherecompression only or expansion only is desired. For example, inbroadcast program distribution, compressionwould be used in order to.restrict the dynamic range to suit the transmission facilities. .If thetransducer control of my invention'is to be:used where the noise levelis high, then expansion may be used. If used on phonograph pickupsexpansion may be used to compensate for the intentional.compression usedin the recording'art. 'When applied as described above the switch 20 .isomitted and the winding A is then coupled directly to the amplifier 26in the proper polarity. The apparatus is then other wise as describedhereinbefore.

In the embodiment of Fig. 1 the sensitivity of the transducer iscontrolled by controlling the magnetic field intensity. Means is alsoprovided to eliminate any effect due to the change in the magnetic fieldintensity. vIn the embodiment of Fig. 2 now to be described, thesensitivity of the transducer is controlled by controlling theefiectiveness of the field onthe element cutting the field. This is doneby :moving the static or mean the microphone.

The ribbon element I2 is acted On by sound waves as before and isconnected by leads l3 to the input of an amplifier asbefore. It is notedthat transformer coupling is not shown in this embodiment, but maybeused I! desired. As a matter of fact, the transformercoupling may beincluded in the amplifier l 8. The diode and its connections to theamplifier l8 may be as described hereinbefore, so that across diode load22 a potential proportional to the magnitude of the current at theoutput of I8 is developed. This potential is supplied by the reversibleswitch 20 to the winding of the driving means M which may be considereda motor. Ihe motor M has a shaft S connected to the mounting element forthe ribbon l2. In the embodiment illustrated, the shaft S is shown asbeing passed through the magnet in in a manner so that its axis is atright angles to the plane of motion of the ribbon l2as acted on by thesound.

In this arrangement, the E. M, F. induced in the ribbon [2 by cuttingthe magnetic field is proportional to the extent of motion. The inducedE. M. F. is greatest when the conductor moves in the entire magneticfield at right angles to the direction of the magnetic flux and isdiminished as the conductor assumes the position in which it moves in areduced magnetic field. The ribbon i2 is mounted on supports X and Ywhich extend in to the axis of rotation and are connected to shaft Sseparated by insulator 36, which also supports and strengthens theribbon l2. The shafts S may pass through insulators 35" in the magnet 4.The ribbon is rotated in one direction or the other by the motor Mdepending upon the position of switch 20, to an extent proportional tothe magnitude of the potential across impedance 22. The motor M may be adevice similar to the moving element in a direct current meter, actuatedby the potential drop across 22. As illustrated the motor M comprises amagnetic structure MS and a coil MC free to move in the magnetic field.The coil MC is connected by an insulator 35' and shaft to the shaft S onwhich the ribbon i2 is mounted for rotation in the field of magnet it.The ribbon 12 may be further supported by the insulator 36. The motor Mrotates the ribbon I2 from its position at which the maximum current isinduced in ribbon .l 2 to a position at which less current is induced inribbon I2 when the magnitude (peak or average) of the current at theoutput of i8 is high and compression or reduction of sensitivity isdesired. The ribbon I2 is then at an angle with respect to the magneticpole pieces and the conductor moves in a smaller magnetic field and theoutput is reduced. Thus, when the switch 20 is adjusted for compression,the conductor I2 is caused by the spring 37 to assume a positionparallel to the magnetic pole pieces in the absence of applied soundwaves. When sound wave energy is applied, the amplified output of themicrophone is rectified by the diode 24, causes a current to flowthrough the motor winding MC and rotate the conductor 12 so that itsmotion causes it to cut a reduced magnetic field, and the'electricaloutput of the system and the sensitivity of the transducer is reduced.

If the switch 26 is reversed, a source 38 and an adjustable resistance39 is included in the motor driving circuit wherein the direction ofcurrent flow has been reversed. The source 38 and resistance 39 is suchas to cause the motor MC to rotate the ribbon [2 on its axis to aposition'whereat it is nearly at right angles to the magnetic polepieces in the absence of output from the amplifier! (i, 9., no soundwaves on the ribbon). The source 38 polarity opposes the polarity ofthepotential developed across'impedance 22x-when soundn.waves areapplied. I However, it will be understood". that the magnitude ofthis'opposing voltagev is adjustable. a

Now when sound waves-strikethe ribbon l2 the voltage developed'acrossimpedance 22increases. The motor MC, actuated by, the potential across22, is caused to move the ribbon 12 to a position at which it becomesnearly parallel to the magnetic pole pieces, and the output of thesystem is expanded. Thus, under expansion condition the ribbon l2, themotor M, and so forth, are set so that the conductor normally assumes aposition nearly at right angles to the pole pieces in the absence ofsound Waves and little electrical output results. This is accomplishedbyarranging bias voltage at 38 to position the conductor E2 at an angleapproaching with respect to the pole pieces, or by supplying a member 5iattached to MC and arranged to strike amechanical stop 43 so that themotion of the conductor is restricted to a position at right'an'glesiothe magnetic pole pieces in case th voltage supplied at 38 is more thanrequired to move MC to said position at which it approaches 90 withrespect to the magnetic pole pieces. The sou'rce38' and resistance 39are means for selecting'si'gnal magnitudes below which there issubstantially no output from the system. This will be clear when it iskept in mind that the current to MC results from the difference in themagnitudes of the opposing potentials at 22 and 38 and that the ribboncannot be made parallel to the magnetic pole pieces until the twopotentials are about equalso that the spring 3'! can move" the coil MCand ribbon 12 into said position. Such operation'is useful ontransducers used, for example, in a noisy location. Then the sensitivityof the system will be greater for desired high level sounds.

The compensation for the effect of the motion of the ribbon l2 and itsmounting means in Fig. 2 is inherent, in that movement of the ribbon bythe motor MC as described above does not produce distortion by changingthe current output. This is because the mounting X, Y is symmetrical andthe current induced by the movement in the two halves of the ribbonmounting are of opposed polarity and balance out in the transduceroutput. This will be apparent by inspection of the ribbon I2 of Fig. 2.When the same is rotated both halvesmove away from the 90 relation, butin opposite directions.

The flux producing field may be of irregular shape, so that theeifectiveness thereof on the moving member l2 increases or decreaseswith movement of the member l2 from the no sound wave position. Thisnon-uniform flux field may be arranged to have either a decreasing flux,as shown in Fig. 2a, or an increasing fiux, as shown in Fig. 2b, atpoints removed from the conductor. Then the potentiometer 30, diode 24,impedance 22, amplifier 26, switch 20 and windings Aand B, all as shownin Fig. 1, are omitted. The magnet in has a cross section as shown inFigs. 2a or 2b. Where the field decreases in intensity, as in Fig.

; 2a, the sensitivity of the transducer is reduced for higher levelsignals and compression takes place. Where the field increases inintensity, as in Fig. 2b, the sensitivity of the transducer is increasedfor higher level signals and expansion takes place.

As stated hereinbefore, my invention is applicable to transducers of anytype, and a further application thereof is illustrated in Fig. 3 of thedrawings. In this figure a transducer or micro- "phone of the cone andvoice winding type-is used. In describing this embodiment reference:char,

7. acters and letters corresponding to those usedin the prior figuresare used in order tosimplifythe description. The microphone 1 or.transducer has acone C responsive-to the soundwaveso The cone C carriesa-fieldvwinding FW mounted between the poles of a magnet 12:". Themoving winding FW cutting the flux field has-induced thereina currentresponding to the, sound Waves, which is fed to the Y amplifier -l 8-asin a the prior embodiment. As in -the prior-embodiment,a-potential'which varies ina mannercorresponding-to the variations inthe output 7 of" I 8 appears inthe impedance-22and is appliedthroughswitch '26 to windings-A-mounted on-theflux field producing magnet 12'.excited by current from 22 is similar to 'the'operation'of thewinding'coil'A in Fig. --1: The current in the winding A 'is in'adirection'to increase the strength of the magnet with increasesinamplitude of" thecurrentoutputof i8 tdacc'omplish expansion: of the saidcurrentsf'to increasethe sensitivity of the microphone; or 'thecurren tin the winding A may'be in the opposite direction to decrease the flux"field'and' the "strength of the magnet, to decrease'the current'at theinputof 18 'when the "current intensity at the "output thereofgoes upandto'thereby compress the signalling the'reof" by decreasing thesensitivity of the microphone, A winding Bin serie's' 'with the WindingFW Selv es the 'sarnef urposje as crewma- 'in'gB'of Fig l'. i

In Fig. I have illustratedfmy improved sensitivity control a's'appliedto transducers or micro'pho'n es 'of the electro static type. In Fig. 4,the microphone A comprises two plates El and P2 spaced fronrea ch otherto f orm, the plates of; a variable condenser,,across impedances 4'! and49. h l m ed cl fis is. a r ss h i p oit e amr li t e 5. he t i l. ropcr s res stt m 4. i a fun n, of th .c pas t the can: den f' L P? an 9 hv lta a o s he. se denser plates. [Ihelatter voltage is-made up of thevoltage o so r e 4 a d th Potent a rQ across the output oi D. C,amplifierjfi. The a p ifier may c m r se the firstst eqfme amplifier:I}; of the prior figures. .flhe' condenser microphone l3! and 1?; ha s afixed plate P'Zand a movable plate Pl. Sound waves impingingjon plate PIvary the capacity of the condenser to sorrgspondingly var i ate of. chge. nd t voltage on the grid of tube 5! and atthe outputof am ifi a d rs .Pfi im da q 2.2-1 To .dws se th sen t the. vel 's ap e rin across theoutput of D. C amplifierjli is so conm e to i pedan '4] s. b op oseteliqlt s o S rc 4 Wi th r s t th tl he. v lta e a p rin a rq t e p ats?tth e e se and I32 is reducedj To increasethe sensitivityof t e r 'nsdqetf hepel ity of th poten 1 d. impedan e 22 sr vle i a i ad sto'thevoltage at 43. The values of these voltages are adiustahleatpotentiometer Q and at source n. th bo ment F 5;. th j 1a ef i m n onafiqofanjlc ma ne mwe- .n dl ev g a Wind n q ct d qth c u te i s o t eswi chi t, n t i ar an ment. as t Sound. Wav i pin e on. the m rop oplate PI itsposition with respectto P2, is changed The operation of thiswvinding A as I ii i . current to the magnet winding, 6| to move theplate P2 toward .or fromthe plate ,Plto increase or decrease thecapacitybetween the p1ates.,,lf the current in the winding 6! isinadirectionto move plate PZYaWay from- PI and reduce thecapacity when thesignal strength increases, compression of the signal takesplace, so thatthe transducers sensitivity is reduced. a If the current throughthewinding 6 l is ina direction to movethe .plate P2 closer to the plate Plto, increase the. capacity when the magnitude-of the voltage appearingat the outputof the amplifier l3 increases, the sensitivity of themicrophone isincreased.

--The core 60 forthe winding 6| may be mounted in any manner, and themountingof the sameis shown schematically in Fig.5. The core 60 is heldby aspring S in such a position that the desired relationshipbetweenplates PI and P2 is maintained.- -Gurrent through the winding 6icompresses oreXpands-the spring S to change the-position-ofthe core :60-and plate P2.

"Whatis claimed-is: I l'wIn signalling apparatus, -a-ribbon-typemicrophone, having-its ribbon in a magnetic field, for" convertingmotion representing signals to electrical currents-an" electricalcircuit connected to the ribbon,'-and means responsive to currents'insaidcircuit for changing the-static position'of the ribbon in themagnetic field. V 2. Atransducer including a movable elementin'a'magnetic field and ma circuit wherein cur rent is produced by'saidmovementyand apparatus including a 'rectifier responsive to theproduce'd currents for producing a control potential. and a'drivingelement-for saidmovable element controlled by said controlpotential for changing the static'position' of said element in the fieldto change'the'sensitivity of said'transducer. i

'35 A transducer'including'a movable element in'a magnetic field and inacircuit wherein current i's produced by said movement, and a motorcontrolled by said'producedcurrent for automatically changing the staticposition of said elemerit in the field in accordance with the strengthof theproduced'current to change the in thafield 'jtochange thesensitivity of said transducer comprising amovable support for saidelement, and means actuated by' the produced for"'controlling theposition of the supy 5.1I n' signalling apparatus, in combination, meansfor converting sound waves to electrical v r at o s .cam ri iaa eelement 6n wh ch t esp n wavesl m l a circuit assodefi d with sa mov b elem nt he e q age is produced of an amplitude which varies with movementof said element, anamplifier cou le with a c r u t, sa m v an Q t ut..whn am l fied e ec l va t ons corresponding to the variations in. thesound waves appear, a rectifierin a circuit including an impedancewherein a voltage is, developed proportional to the magnitude of the.signaLin theoutput. of the amplifier, and electrical .or mechanicalmeans excited by-said last.named voltage forregulating the sensitivityof the converting means..,

6..In signalling?apparatualin combination,

means forconverting. soundwavesto electrical variations comprising meansfor producing a magnetic field, a movable element in saidfield on whichthe sound waves impinge, a circuit associated with said movable elementwherein a voltage is produced of an amplitude which varies with movementof said element, and an amplifier coupled With said circuit, saidamplifier having an output wherein amplified electrical variationscorresponding to the variations in the sound waves appear, the saidmagnetic field being irregular so that the intensity thereof changeswith movement of said element.

7. In signalling apparatus, in combination, means for converting sound'waves to electrical variations comprising a movable element on whichthe sound waves impinge, a circuit associated with said movable elementwherein a voltage is produced of an amplitude which varies with movementof said element, an amplifier coupled with said circuit, said amplifierhaving an output wherein amplified electrical variations correspondingto the variations in the sound waves appear, a diode having an anode anda cathode, means including an impedance connecting the cathode and anodein shunt to a part at least of the output of said amplifier whereby avoltage is developed in the impedance proporl 0 tional to the magnitudeof the signal in the output of the amplifier, and electrical ormechanical means excited by said last named voltage for regulating thesensitivity of the converting means.

8. A transducer including a movable element in a magnetic field and in acircuit wherein current is produced by said movement, means forautomatically changing the static position of said element in the fieldin accordance with the strength of the produced currents to change thesensitivity of said transducer, and means for compensating the effect ofchanges in the static position of said element on the produced current.

GEORGE K. GRAHAM.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,519,211 Martin Dec. 16, 19241,565,548 Clark Dec. 15, 1925 1,967,125 Miller July 17, 1934 2,113,219Olson et a1. Apr. 5, 1938 2,309,109 Hathaway Jan. 26, 1943

